Movement of Missiles by North Korea Worries U.S.

WASHINGTON — The discovery by American intelligence agencies that North Korea is moving mobile missile launchers around the country, some carrying a new generation of powerful rocket, has spurred new assessments of the intentions of the country’s young new leader, Kim Jong-un, who has talked about economic change but appears to be accelerating the country’s ability to attack American allies or forces in Asia, and ultimately to strike across the Pacific.

The new mobile missile, called the KN-08, has not yet been operationally deployed, and American officials say it may not be ready for some time. But the discovery that the mobile units have already been dispersed around the country, where they can be easily hidden, has prompted the White House, the Pentagon and intelligence agencies to reassess whether North Korea’s missile capabilities are improving at a pace that poses a new challenge to American defenses.

On Thursday, speaking in Italy, the departing defense secretary, Leon E. Panetta, broke from the usual Obama administration script — which is to write off North Korea as a broke and desperate country — and told American troops that he was increasingly worried about another, longer-range North Korean missile, one that was successfully tested last month and reached as far as the Philippines, and could lob a warhead much farther.

“Who the hell knows what they’re going to do from day to day?” Mr. Panetta said. “And right now, you know, North Korea just fired a missile. It’s an intercontinental ballistic missile, for God sakes. That means they have the capability to strike the United States.”

After he spoke, Pentagon officials said Mr. Panetta did not mean to imply that North Korea could now hit the continental United States, although intelligence and military assessments have said that Hawaii is within range. But the North has made progress toward its goal of fielding a missile that could cross the Pacific, a goal the previous defense secretary, Robert M. Gates, warned at the end of his time in office could be fulfilled by 2016.

An intensive study of the long-range missile test-flight conducted by North Korea last month, one administration official said, found that it was “largely a success, if you define success as showing that they could drop a warhead a lot of places in Asia.”

The more immediate mystery for the administration, however, is what North Korea may intend with the intermediate-range KN-08, which was first shown off by the North in a military parade last April. At the time, many analysts dismissed it as a mock-up. In fact, it has never been test-flown. But parts, including the rocket motors, have been tested separately, according to officials familiar with the intelligence reports, who described the missile developments on the condition of anonymity because they were not authorized to discuss the assessments.

Officials familiar with North Korean missile technology say the KN-08 weapon is designed with a range capable of striking South Korea, Japan and parts of Southeast Asia — although with uncertain accuracy.

North Korea is aware that it is a focus of American spy satellites, so the decision to roll the missile around the country to potential deployment sites might well have been partly motivated by a desire to send a message to the United States, or at least to get Washington’s attention — which it did. Officials said that North Korea’s advancements in missile technology were among the most significant reasons that Mr. Panetta, as he approached the end of his tenure, had spent so much time in Asia. Much of his effort has been aimed at spurring the development of a regional missile defense system to be deployed with allies, particularly Japan and South Korea.

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There is no evidence that the KN-08 has been fitted with a nuclear warhead. While North Korea conducted nuclear tests in 2006 and in 2009 — just months after President Obama took office — American intelligence officials have said that the North has not miniaturized a nuclear device small enough to be fitted as a warhead atop its missiles. Some believe that may be the goal of its next test — and perhaps, some intelligence reports speculate, of continuing cooperation on missile design between Iran and North Korea. The Iranians, one official noted, “are grappling with the same issues.”

In fact, much remains uncertain about North Korea’s new missile. There was no question where the mobile launching trucks that carried the missile came from: they are Chinese, and almost certainly imported in violation of United Nations sanctions against the North. The new missile, like most in the North Korean arsenal, appeared to be based on Russian technology.

The missile developments are among a number of steps that have convinced American officials that, a year after his ascension as the third generation to inherit the role as North Korea’s dictator, Mr. Kim is proving as confrontational with the West as his father and grandfather. American specialists also warn of the prospect of a third nuclear test, for which preparations are evident.

For the Obama administration, whose last diplomatic effort with the North ended in failure nearly a year ago, the steps are reminders that everything they have tried in the past four years to lure the country out of isolation — or at least contain its nuclear and missile programs — has largely failed.

If nothing else, however, the missile efforts in the North have spurred American efforts to build a network of antimissile capabilities across Northeast Asia. Japan already has one American X-band radar, officially known as the AN/TPY-2, which is a central element in a complex technical architecture for identifying ballistic missiles and coordinating a response by interceptors. And last September, during his travels in the region, Mr. Panetta and his Japanese hosts announced a major agreement to deploy a second advanced missile-defense radar on Japanese territory.

A version of this article appears in print on January 18, 2013, on Page A3 of the New York edition with the headline: Movement of Missiles by North Korea Worries U.S. Order Reprints|Today's Paper|Subscribe